1. Field of the Invention
The present invention relates to rolling bearings used, for example, with automotive vehicles, agricultural machinery, construction machinery and iron-and-steel machinery and, more particularly, to long-lived rolling bearings required for use in a transmission and engine.
2. Description of the Related Art
A rolling bearing is in a severe use under high bearing loads so as to experience a repeated shearing stress. In order to endure the shearing stress and secure rolling contact fatigue life (hereinafter also referred to as rolling life or fatigue life), the rolling bearing which is made of high-carbon chromium steel (e.g., JIS-SUJ 2), then quenched and tempered so as to have a 58-64 H.sub.R C surface hardness, has been used. On the other hand, a case hardened steel rolling bearing has been made of a low-carbon case hardened steel of a good hardenability, e.g., JIS-SCR420H, -SCM420H, -SAE6820H or -SAE4320H, since a hardness curve of the case hardened steel rolling bearing must be determined in accordance with an internal shearing stress distribution created by a contact bearing. Then, this case hardened steel rolling bearing has been carburized or carbonitrided, quenched and tempered and thus has secured a required life so that the inner and outer races of the case hardened steel rolling bearing have 58-64 H.sub.R C surface hardnesses and 30-48 H.sub.R C core hardness.
However, since it is desirable that a rolling bearing have an increased fatigue life at the present time, a more severe condition, e.g., foreign matter entrained in a bearing lubricating oil, can damage the rolling element and the inner and outer races of the rolling bearing. Thus, a prior-art bearing hardness has been occasionally insufficient.
Thus, it is necessary that the surface hardness of a rolling bearing be increased. There is a prior-art process for increasing the surface hardness of a rolling bearing in which the rolling bearing is made of, e.g., a precipitation hardened tool steel (e.g., JIS-SKH or -SKD) including added carbide forming elements and precipitating a multitude of carbides (see pages 780-797 of Metal Manual, 3rd edition, edited by Japanese Metal Institute).
The rolling bearing made of the above precipitation hardened tool steel has advantages in that it has an increased rolling contact surface hardness and resists the occurrence of an impression caused by a foreign matter entrained in lubricating oil. On the other hand, it has a drawback in that a coarsening of a precipitating carbide which may be caused by the content of an alloying element to produce the carbide creates stress concentration in a part surrounding the carbide and propagates from this part into a flaking thereby reducing the fatigue life of the rolling bearing.
Limiting the content of the retained austenite in the surface layer of a rolling bearing within a predetermined range can moderate stress concentration at the edge of an impression by the foreign matter to increase the fatigue life of the rolling bearing (see Japanese patent application SHO 62-209167). However, an automotive rolling bearing used under a high bearing load, for example, is required to have a greater increase in its fatigue life.
In order to overcome these outstanding problems, the present invention has an object to provide a longer-lived rolling bearing than prior-art rolling bearings.